<p>by Rosanna C. T. Wright, Simon C. Lovell, Anna Richmond, Christian Harrison, Eleri Ashworth, Aras Kadioglu, Joanne L. Fothergill, Ville-Petri Friman, Edze R. Westra, Michael A. Brockhurst</p> Only a fraction of bacterial genomes encode CRISPR-Cas systems, but the selective causes of this variation are unexplained. How naturally virulent bacteriophages (phages) select for CRISPR immunity has rarely been tested experimentally. Here, we show against a panel of genetically and functionally diverse virulent phages that CRISPR immunity was not universally beneficial to <i>Pseudomonas aeruginosa</i>, and its fitness effect varied strongly between phages in predictable ways. In addition to mechanisms known to alter the effectiveness of CRISPR immunity, such as encoding a matching spacer or a protective nuclear shell, we show that the fitness effect of CRISPR immunity negatively associated with the probability of evolving receptor-based resistance to the phage via spontaneous mutation. The supply of resistance mutations differed strongly between very closely related lipopolysaccharide-binding phages and was associated with variation at the C-terminus of the tail fibre protein altering residues involved in hydrogen bonding and the predicted binding site. Our results show that CRISPR immunity is more beneficial against virulent phages that are harder to evolve resistance to via receptor mutations, suggesting that virulent phage community composition and diversity will be important drivers of the prevalence of CRISPR immunity.